In using chromatographs, whether gas, liquid or supercritical fluids are employed it is sometimes necessary to couple capillary tubes together. In an injection port, for example, the tube into which the syringe needle is thrust may be larger than the capillary column to which it is coupled. It may also be necessary to couple the output end of a separating column to a tube that is connected to a detector.
One significant disadvantage of presently known coupling devices is that they have a dead space communicating with the ends of tubes being coupled. A portion of the sample fluid emerging from the end of one tube quickly finds its way into the dead space but a relatively long time is required for it to enter the other tube. Therefore the concentration of the sample fluid emerging from the other end of the latter tube increases rapidly to a maximum value and then slowly decays to zero so as to cause a phenomenon known as tailing. As those skilled in the art are aware, the tail of one sample can enter a detector at the same time as the main portion of a succeeding sample fluid so as to make it difficult to separate the response to the detector to one sample from its response to the other.
Another significant disadvantage of presently known coupling devices is that the fluid flowing through the tubes can be degraded by contact with large areas of surfaces of the device other than the surfaces of the tubes.